This invention relates to an incremental encoder comprising a rotor peripherally possessing a circular succesion of windows, and at least one reader formed from a pair of reading units for reading the passage of the windows, said units comprising a light emitting diode, a screen provided with windows, and a light-sensitive semiconductor (phototransistor). Encoders are electro-optical instruments for measuring the angular rotation of a shaft by means of a series of pulses. they are of very widespread use, and their operation is known. They comprise: a rotor provided peripherally with a large number of radially elongated small-dimension windows, a monochromatic light emitting diode, a screen disposed in front of the diode and provided with several windows equal to those of the rotor, and a phototransistor which receives the light emitted by the diode and passing through the screen and rotor windows, when the rotor windows at least partly face those of the screen. The phototransistor emits an electrical signal which is a function of the light received. If the rotor windows do not at least partly face the screen windows, the light does not pass and the phototransistor does not conduct. For each complete revolution of the rotor the phototransistor conducts a number of times equal to the numer of rotor windows. The distance between the centres or centre lines of two adjacent windows represents the pitch, and the number of windows is equal to the number of signals emitted per revolution. If each window has a width of one half the pitch, the pattern of the phototransistor current is substantially sinusoidal during the rotor rotation, and during one revolution the number of sinusoids is equal to the number of rotor windows.
Encoders also comprise a second diode-phototransistor-screen assembly, in which the screen is displaced by one half of a window (i.e. 90 electrical degrees) with respect to the screen of the first assembly, so that in this second assembly the signal from the relative phototransistor is displaced in phase by 90.degree. from the signal of the first during rotation. The two output signals from the two phototransistors are converted into two square waves, which are such that when the rotor rotates in one direction the first wave ascends when the other is low, whereas when the rotor rotates in the opposite direction the first wave ascends when the other is high. It is thus possible to differentiate between the pulses generated in one direction of rotation from those generated in the reverse direction of rotation.
Of all the pulses which form the square waves, both the ascending wave fronts and the descending wave fronts are counted to finally obtain four pulses for each rotor window.
The most important characteristic of an encoder is the accuracy with which it indicates the angle of rotation undergone by the shaft associated with the rotor.
It is well known that in order to indicate a value with great accuracy, many digits and thus many rotor windows are required. Int he current state of the art, the maximum number of windows in a ring of 50 millimetres diameter is 3000, to obtain 12,000 pulses per revolution counting all four wave fronts. In such an encoder, the width of each window is 26 microns and the displacement of one half of a window is therefore 13 microns.
Such a rotor, shown diagrammatically in plan view in FIG. 1, has two defects, the first being the off-centering, and the second the non-uniform distribution of the windows.
These two defects cannot be overcome. this is because off-centering always exists in any rotary member, and non-uniform window distribution necessarily results from the photographic and chemical method generally used for forming the windows and which is influenced not only by the imperfections in the initial design, but also by the photograph (such as parallelism of the design and the photographic plate) and by the lens.
FIG. 1 shows, of a known encoder, the rotor with two reading units. In this rotor, indicated by R, the windows are indicated by 11, the first reading unit by 12, and the second unit with its screen displaced through one half of a window by 14. The theoretical centre of the rotor is indicated by 15, and the actual centre, which is effectively off-centre, by 13. In FIG. 1 it can be seen that if each window is assumed to be 26 microns wide and the screen of the unit 14 is assumed to be displaced by one half of a window, ie by 13 microns, even if the windows were all absolutely perfect a distance between the centres 15 and 13 of only 6.5 microns would be sufficient to displace the unit 14 through one half of a window, thus making it useless. The displacement of 90 electrical degrees would thus become either 0.degree. or 180.degree..
Taking into account the additional fact that the non-uniform distribution of the windows can reach 10-20 microns, it is immediately apparent that the assembly cannot operate correctly. Encoder manufacturers have sought to avoid these diffiulties by mounting the two reading units very close together, as shown in FIG. 2. In this manner, neither the off-centering nor the non-uniform distribution of the windows should alter the phase difference between the two signals by very much. However, both the off-centering and the non-uniform distribution still exist and, although to a lesser extent, they still influence the inidication of the shaft angle, which is the very quantity which should be accurate.
For completeness, it must also be stated that in encoders each reading unit is balanced by a further unit displaced through 180 electrical degrees from the first. The displacement in electrical degrees indicates the phase angle between the output signals of the phototransistors of two reading units. A displacement of 180 electrical degrees signifies that these signals are in opposition. This is obtained by displacing the screen of one unit by on half of a pitch, ie by one window, relative to the screen of the other unit. The known purpose of this 180.degree. displacement is to enable a better reading to be fed to a voltage comparator in order to obtain the square wave signal. The two reading units displaced in phase by 180 electrical degrees form a "reader".